(a) Field of the Invention
The present invention relates to a steel sheet used under a high temperature and corrosion environment, and in particular, to a steel sheet for an automotive muffler, which is excellent in corrosion resistance against condensed water generated in the automotive muffler, impact resistance, and a product's service life.
(b) Description of Related Art
An automotive vehicle or electronic appliance has a variety of components formed of a steel sheet. Many of the components are used under a high temperature and corrosion environment.
A muffler of an exhaust system of the automotive vehicle may be exampled as the component used under the high temperature corrosion environment.
The muffler functions to cool and exhaust high temperature/high pressure combustion gas and reduce the exhaust noise. The muffler includes a muffler body, an exhaust pipe connected to the muffler body, and a flange for coupling the exhaust pipe to the muffler body. Although there may be a difference according to a kind of the automotive vehicles, a plurality of partitions and a plurality of small pipes are generally installed in the muffler body in order to reduce the noise generated in the muffler body.
The automotive muffler is not used under a constant temperature environment but under an environment where the temperature increases and decreases according to the driving state of the automotive vehicle. In addition, combustion gas generated from an engine passes through the automotive muffler, in the course of which the combustion gas reacts with moisture in the muffler to generate condensed water. The condensed water contains high corrosive combustion gas ions such as SO32−, NH4+, SO42−, Cl−, NO2, or NO3−.
When the automotive vehicle is run for a long time, an internal corrosion is generated in the muffler due to the condensed water generated in the muffler. In addition, an external corrosion is generated on the muffler due to, for example, a deicing agent such as calcium chloride.
Due to the above reason, the automotive muffler must be formed of a material that is excellent in corrosion resistance, heat resistance, and impact resistance.
A steel sheet coated with aluminum and a stainless steel sheet are well known as a typical steel sheet used for producing the automotive muffler.
The steel sheet coated with the aluminum is not appropriate for the muffler material since the aluminum is costly compared with the steel sheet. In addition, when the steel sheet coated with the aluminum is used for a long time, the aluminum coating layer is corroded and thus the steel sheet corresponding to the corroded portion of the aluminum plaiting layer is quickly corroded. In order to solve this corrosion problem, there is a method for increasing a thickness of the aluminum coating layer. However, as the thickness of the aluminum coating layer increases, the production costs increase. Furthermore, there is a technical limitation in increasing the thickness of the aluminum coating layer to a certain level. Therefore, the steel sheet coated with the aluminum has many problems in terms of the corrosion resistance and the production costs to be used as a material for producing the automotive muffler.
Although the stainless steel sheet that is another material for producing the automotive muffler is known that it is relatively excellent in the corrosion resistance, the stainless steel sheet is costly as it is. In addition, since the automotive muffler is generally used under an environment where the variation of the temperature fluctuates from a high temperature to a constant temperature or from a constant temperature to a high temperature, the stainless steel sheet encounters a high temperature corrosion resistance problem of itself.
In order to solve the problem, the improvement of a property of the coating layer formed on the steel sheet, the change of a component of the stainless steel sheet, or the stainless steel sheet coated with the aluminum has been proposed.
Japanese laid-open patent No. 1999-269605 discloses a stainless steel sheet coated with aluminum. A composition of the stainless steel includes less than 0.004% by weight of C, 0.04 to 0.08% by weight of P, equal to or less than 0.01% by weight of S, 0.02 to 0.10% by weight of Ti, and equal to or less than less than 0.003% by weight of N. Zn—Al alloy including 30 to 70% by weight of Al, 0.5 to 2.5% by weight of Si, and a remainder of Zn is coated on one side or both sides of the steel plate.
However, the steel sheet coated with the Zn—Al-based alloy of the patent still has a problem that the corrosion resistance thereof is not sufficient.
Japanese laid-open patent No. 1990-270521 discloses a stainless steel that is coated with aluminum to enhance the corrosion resistance. Japanese laid-open patent No. 1976-136792 discloses a steel sheet whose components are adjusted to improve the welding property.
Since the steel sheets of the above two patents still contain a large amount of expensive alloy iron such as Ni-based alloy iron or Cr-based alloy iron, it has a problem in that the production costs increase.